Stevia (also called sweetleaf, sweet leaf or sugarleaf) is a genus of about 150 species of herbs and shrubs belonging to the Asteraceae (sunflower) family, native to subtropical and tropical South America and Central America (north to Mexico). As a sweetener, stevia's sweet taste has a slower onset and longer duration than that of sugar, although some of its extracts may have a bitter or liquorice-like aftertaste at high concentrations. With its extracts having up to 300 times the sweetness of sugar, stevia has garnered attention with the rise in demand for low-carbohydrate, low-sugar food alternatives. Stevia also has shown promise in medical research for treating such conditions as obesity and high blood pressure. Stevia has negligible effect on blood glucose, therefore it is attractive as a natural sweetener to diabetics and others on carbohydrate-controlled diets. However, health and political controversies have limited stevia's availablility in many countries; for example, the United States banned it in the early 1990s. Stevia is widely used as a sweetener in Japan, and it is now available in the US and Canada as a food supplement, although not as a food additive.